ink disk polishing?

This may be a naive question, bear with me!
I’m having an engineer re-grind the ‘dished’ ink disk from my Model no.3 to remove the slight flat spot in the centre that is making even inking impossible.
The engineer is experienced, and I’ve emphasised the need for a perfectly flat and smooth surface. He’s pointed out that the disk has originally been ‘polished’. My question: is it essential to re-polish the disk to this mirror-like surface? Will the lack of a polished surface adversely affect the transfer of ink to the rollers?
Thanks for any advice on this point.
Geoff

PROBABLY the smoother the disk the better and more even the coat of ink. But what the difference is between smooth and REALLY smooth is possibly more academic than a practical limitation. You could always smear some grinding paste on a thick sheet of glass and give the disc a polish that way if you were really cheap. I mean concerned.

I assume your engineer is surfacing the disk on a lathe. If done carefully and correctly it will have concentric machining marks. These do not need to be polished out if you cannot catch a finger nail on the marks.

Thanks both for your helpful advice. Ive been waiting for the ink disk to be returned from the engineer before replying. In the end he said he could re-polish it as well, so that’s what I had done. Apparently it was 16 thou out of true, with a doughnut-shaped ‘flat spot’ around the centre. It is now flat to within a couple of thou, which I think should give me much more consistent inking.
But, out of interest, do you have any ideas how this ‘flat ring’ could have been caused?

From time to time we come across ink disks that are not flat. We make them flat and perpendicular to the stub shaft. Some run-out is bound to be present but has no effect on disk performance. In almost all instances this result will demand a bushing (shim) to be installed on the disk shaft to raise the disk back to its correct height so the ink rollers transition from rails to disk with a slight amount of deflection, at the very edge of the disk. This of course enables the rollers to experience total contact with the inked disk. I would conclude that poor quality control during manufacturing, a failed attempt to flatten a disk in its past or a reproduction is the likely cause of your flat spot.
When turning a disk on a lathe great care must be used to avoid deflecting the disk. Surface grinding is an approach that in theory would produce the best results. Good luck finding a grind shop that will service a single part. Set up fees and and an understanding of the desired results are obstacles to utilizing this method.
It bears noting that slight pitting is common especially if composition rollers were, at some time, left upon the disc for an extended period of time. Some ink disks are too thin to reface and must be left alone despite the desire to reface. Lastly many older ink disk castings have inclusions in the iron and surface when attempting to reface. When a cutting tool hits an inclusion, like sand, the result can become worse than the original malady one had hoped to cure.

Hi Tom
Thanks for all this interesting information, and the advice to install a shim on the stub shaft - I’ll certainly do that.
In fact my disk was re-ground on a surface grinder, but as you surmised, finding someone to do it was not easy! I started with the engineer who re-ground the cylinder head on my nephew’s vintage MG; from there I was referred to a second shop, who didn’t have a grinder large enough, but referred me on to a contact of theirs, who’d retired but sent me to yet another engineering shop where I found someone who was sufficiently intrigued to tackle the job. They mounted the disk in a chuck to keep it absolutely horizontal, and a concern was that it might vibrate when the cutting tool first made contact. I had emphasised that they shouldn’t risk damaging the disk, so there were a couple of days’ nervous waiting to hear if they had managed the job successfully. The re-grind and polish came in at well under three figures (sterling) so I am pretty happy.
Geoff

I once used my blast cabinet to “resurface” an ink disc - I will never do that again. Machine work is the only way to go. I also find grinding non-heat treated metal scary. I say use a lathe but it too is also scary. My 6x10 Kelsey disc is very much as described here. I use a steel bushing (not bronze)on my Kelsey. Heat treated washers if you cannot find a bushing. The Kelsey/Sigwalt discs in my opinion are way to thin to machine compared to a Craftsman Superior which I have and that disc is machined flat.